When Mutsuro Yuji, chief of the central sewage plant here in this southern Japanese city, first heard about the idea of making hydrogen from biogas — the combination of methane and carbon dioxide produced by the breakdown of stinky matter — he was skeptical. “I thought it was a joke,” he says.

But after a $12-million investment from Japan’s government, plus research, engineering, design and building work by Mitsubishi, Toyota and Kyushu University, Yuji is no longer laughing. Starting late last year, drivers of vehicles like the Toyota Mirai and Honda Clarity have been able to roll up to the sewage plant and power up their hydrogen fuel cell cars at what you might call the world’s first toilet-to-tank filling station.